Fix a locking bug in skip_sequential_io introduced in the ulocks commit.

Summary: We were trying to acquire the ioctl_lock() again in the same thread.
It was acquired in flashcache_map(), which calls into flashcache_uncacheable()
which calls skip_sequential_io(). The locking changes tried to re-acquire the
spinlock in skip_sequential_io() again.

Test Plan:

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Task ID: #

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